Actually, I just thought of a more simple test for this.
I realised that the Lineage (16.0) recovery system supports the Pro1's hardware keyboard: when I boot into recovery, I can navigate through its menu using the up/down-arrow, return, and backspace keys on the keyboard.
Could you check if this works on your phone? If yes, this would clearly demonstrate that the keyboard itself is fine. If not, I think it would be strong indication of a hardware failure and your chances of fixing things by reinstalling the system would be slim ...